1. Field of the Invention
The present invention relates to a dichroic dyes and composition thereof, and in particular relates to a dichroic dyes used in photoelectric products and microencapsulated liquid crystal compositions employing the dichroic dye.
2. Description of the Related Art
There is an increasing need for reflective liquid crystal displays for use in information products, such as E-books and PDAs, because they consume less power and are thin and light. Currently, the guest-host display mode is applied in reflective liquid crystal displays, dye being “guest” and liquid crystal being “host”. The liquid crystal display effects are generated by the anisotropically optical absorption by dichroic dye which is dissolved in the mixture of nematic liquid crystal and an optically active substance. This display mode employs absorption or no absorption for visible light by dye, exhibits bright and dark characteristics corresponding to the variation of electric field and can replace reflectors. The viewing angle of this type of reflective liquid crystal display is improved and the brightness is also increased. When used, only environmental light is required, but not back light, therefore power consumption can be greatly reduced.
In guest-host display mode, dichroic dyes are used and must exhibit good dichroism, which can be quantified by the orientational order parameter (SD) or dichroic ratio (N) of the dye molecules. The contrast is higher when the values of N and SD are higher. Moreover, the dye molecules in the display devices tend to deteriorate due to long term exposure to back light irradiation or environmental light (such as sun light), and the display quality is affected adversely. Accordingly, it is desirable for the dyes to have good photostability and heat stability for enhanced durability. Furthermore, the solubility of dichroic dye in liquid crystal is generally 1%˜10% based on the weight of liquid crystal. The dichroic dye rotates as the liquid crystal rotates in accordance with the increase of applied electric voltage. To increase the contrast of the display, an increase in solubility and load of dichroic dye in liquid crystal is desirable to fulfill the prerequisite of avoiding reduced brightness and increased threshold electric voltage.
However, few conventional dichroic dyes meet all the requirements mentioned above. Conventional dichroic dyes, those which meet the basic requirements for dichroism and coloration, often have poor processability because of poor solubility or short product life because of poor photostability. For example, the photostability and heat stability of general azo dyes are poor, but those of anthraquinone dyes are relatively stable due to the hydrogen bond between molecules. The mono-azo dye having heterocyclic julolidine or thiazole group has relatively high photostability, but reduced solubility. U.S. Pat. No. 4,304,683 discloses an anthraquinone dye having a long chain soft hydrocarbyl group, which has an asymmetric molecular structure, but poor solubility.
Therefore, it is necessary to develop a dichroic dye with superior dichroism and solubility for liquid crystal.